1. Field of the Invention
The present invention relates to an elastomeric insulating composition for solid propellant rocket motors.
2. Description of Prior Art
In general, a propellant of solid rocket motors is composed of various kinds of high-energy materials and oxidizers. When ignited the propellant releases huge amount of gases and heat energy that propels the rocket to move forward at very high speed. However energy accumulated therein and temperature in the motor case can rise to 2400° C.˜3700° C. in a very short time. Such high temperature may destroy the rocket motor case if it is not properly protected. Therefore an insulator is used in solid propellant rocket motors to protect the motors from damage.
Insulators usually contain fillers such as asbestos, aluminum hydroxide (Al(OH)3), antimony oxide (Sb2O3) and silicon dioxide (SiO2), etc. to increase the ablative resistance of the insulators. These fillers are added to a rubber compounding recipe and the resultant rubber stock is thereafter calendered into thin sheet. The uncured rubber sheet is applied to the inside wall of the motor case by a suitable method, such as an inflatable mandrel technique, and cured therefore to form the insulator layer.
U.S. Pat. No. 3,347,047 (1967) described a rubbery insulator containing 3 phr to 200 phr (parts per hundred weight) asbestos in rubber, which fulfills inflatable mandrel technique requirements in practical application work. A corresponding commercial product designated V-44 with a specific gravity 1.28 gr/cm3 made by the Aerojet Company in the United States is made from NBR (nitrile butadiene rubber) and asbestos fiber. This product has been applied to tactical missiles such as the Patriot™ of the United States and the Gabriel™ of Israel.
U.S. Pat. No. 4,501,841 (1985) and U.S. Pat. No. 4,878,431 (1989) use polyaramid fiber and inorganic fillers in an insulator compounding recipe. Notable increasing in ablation resistance was disclosed. However, the specific gravity of this ablative material ranging from 1.4 gr/cm3 to 1.5 gr/cm3 is considered to increase the total weight of rocket motor and, therefore, its firing range is minimized.
U.S. Pat. No. 5,821,284 (1998) disclosed the synergistic effect of ammonium sulfate and antimony oxide that were used in an insulator composed of EPDM (ethylene-propylene diene terpolymer) rubber and polyaramid fiber. The ablation rate of the insulator thus prepared is significantly reduced because of such synergistic effect. The specific gravity of the insulator may be as low as 1.18 gr/cm3. According to the patent, versatile application methods and better mechanical properties of the insulator are declared.
U.S. Pat. No. 5,498,649 (1996) disclosed an insulator made from EPDM (ethylene-propylene diene terpolymer) rubber containing polyamide and maleic anhydride with a low specific gravity. However, there is no data to support the ablative resistance character of this insulator in this patent. The disclosed insulator of this patent is compounded in a thermal plastic polymer that is totally different from the thermosetting rubbers of the general insulator formulation used.
Antimony oxide has long been used together with chlorinated flame retardant to form a synergistic effect in insulator formulations and is reported to have superior ablation resistant effect. However, antimony oxide releases toxic gas when burnt and that gas is harmful to the environment. Therefore, it is desirable to provide an improved composition of the insulating materials to mitigate and/or obviate the aforementioned shortcomings.